I'm 18- so although I feel qualified to give an answer; thats a very broad question, and it can be a daunting task for any writer. Even though sometimes I reflect on a conversation I've had with my friends, and stand back and realise how it would be total nonsense to one of my parents- there are countless niches of slang for teens, all over the English speaking world.
Where do your teenagers live? What kind of things are they into? Who do they associate with? Who would they never associate with? What irritates them? What do they find cool?
The less ambitious and concerned you are with trying to make them sound like "teens" the better I'd say, it could come off as laughable. If you have/know any kids- just think about how they would respond to the situations in your writing, what would they say?
Watch some MTV (that may sound ridiculous but it all depends on what kind of teens you hope to depict, would they watch 'My Super Sweet 16' or some teenaged pregnancy trash? I find my generations use of language to be massively influenced by whatever's on TV).
Remember that where/when they come from is hugely significant. One of my favourite depictions of teens and their dialogue is Bret Easton Ellis' Less than Zero (but even thats quite a niche- 80's Californian late-teens/young adults, shades of valspeak etc)